1. Field of the Invention
This invention relates to a device for monitoring time-temperature histories in which a vapor is allowed to contact an indicator to cause a color response, by permeating through a vapor-permeable barrier, whereby a color response occurs only after a definite characteristic induction period.
2. Brief Description of the Prior Art
A host of perishable products such as frozen foods, blood, vaccines and the like require a means for monitoring time-temperature shelf life histories.
Various indicators have been developed in the prior art to meet this need with varying success.
Indicators are known that use a liquid or liquid vapor for activating a color response indicating that a predetermined shelf life has expired. For example, U.S. Pat. No. 3,844,718 discloses a defrost indicator which is activated by contact of water vapor with a water-soluble ink supported on a hygroscopic substrate.
U.S. Pat. No. 3,768,976 discloses a t-T indicator that depends upon the rate of permeation of gaseous oxygen through a polymer envelope containing an aqueous solution of a red redox dye. Upon oxidation, the red dye turns colorless, indicating that the perishable has been exposed to too high a temperature for too long a time.
U.S. Pat. No. 3,615,719 discloses a temperature indicator in which a frozen liquid is separated from an indicating layer by a liquid-soluble barrier. When the frozen liquid thaws, a time delay is introduced by the rate of dissolution of the liquid soluble barrier. Only upon dissolution of this barrier does color indication occur.
U.S. Pat. No. 3,501,297 discloses a mixture of diacetylenes, irradiated by ultraviolet radiation producing a blue-bronze color, capable of being converted by contact with warm ethanol vapors to a red color. However, such a color change by itself, is insufficient to suggest its application as a practical time-temperature history indicator, since the color change may simply indicate that a particular temperature has been exceeded, without an indication either of the length of time that temperature has been exceeded or of the time-averaged exposure at higher temperatures.
There is a continuing need for improved indicators for monitoring the time-temperature histories of perishable products.
Known successful devices usually require either liquids or gases for initiation of the color response. A device requiring vapor would be relatively less expensive since less liquid would be required to activate the color response. However, it is known that contact between vapor and a solid, capable of undergoing a color response upon contact with said vapor, usually produces a color response almost immediately. The prior art does not suggest a device based solely on the interaction between a vapor and solid, in which a characteristic induction period prior to color development can be varied. Such a device could be effectively used for monitoring a range of time-temperature histories of perishable articles.